Motion compensation is a major source of coding gain where most of the compression comes from for video. Current motion compensation approaches in existing standards use short-tap fixed interpolation filters. Conventional encoder and decoder techniques are well known and of reasonable cost for the current approaches. Research has shown that using more taps and making the taps programmable, rather than fixed, can lead to impressively increased compression at a cost. Future international compression standards will likely use such techniques, despite the increased cost, to achieve sufficient compression gains to justify the use of the new standard in existing application domains.
Referring to FIG. 1, a diagram of current approaches for motion compensation interpolations are shown. The current approaches (i.e., H.264 and the proposed H.265) use long sets of filter taps in hierarchical interpolation techniques. The hierarchical interpolation techniques mean that half-pel interpolated values are first interpolated, then quarter-pel interpolated values are derived from the interpolated half-pel values and then eighth-pel interpolated values are derived from the interpolated quarter-pel values. An advantage of the hierarchical approach for programmable filter taps is that typically most of the gain can be derived by only making the first level of interpolation programmable and leaving the other levels fixed so that no filter taps need be transmitted for upper levels of interpolation.
A disadvantage of the existing approaches is that by using multiple levels of interpolation and deriving higher-level interpolated positions from lower-level positions, the upper interpolated levels cannot typically use just bilinear interpolation in order to get the level of performance specified for future video standards. To achieve the specified performance, the higher level interpolation must use more than two taps. As such, a bus-bandwidth used for motion compensation can be significantly increased.
A reason that the hierarchical approach makes economic (i.e., cost-benefit) sense in the current H.264 video coding standard is because the filter values for the half-pel interpolation are fixed and simple, permitting a low-complexity implementation for digital signal processors. Because the second interpolation layer is bilinear, no additional bus bandwidth is consumed generating quarter-pel interpolation values. However, when a second or a third interpolation layer technique utilizes more than two taps (for increased prediction accuracy and coding gain) the hierarchical approach has a real bandwidth penalty increasing cost over a direct interpolation approach.